HR: 1330h
AN: T52A-0243    [PDF]
TI: In-slab Stresses and Damaging In-slab Earthquakes in Southwest Japan
AU: * Wada, I
EM: ikukow@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, PO Box 3055 STN CSC, Victoria, BC V8W 3P6 Canada
AU: Wang, K
EM: kwang@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria, PO Box 3055 STN CSC, Victoria, BC V8W 3P6 Canada
AU: Wang, K
EM: kwang@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, PO Box 6000, Sidney, BC V8L 4B2 Canada
AU: Ishikawa, Y
EM: catfish@wa2.so-net.ne.jp
AF: Meteorological Research Institute, Japan Meteorological Agency, 1-1 Nagamine, Tsukuba, Ibaraki, 305-0052 Japan
AB: The Philippine Sea (PHS) plate subducts obliquely beneath Southwest Japan at a concave seaward plate boundary. The stress regimes of both the overriding and downgoing plates provide critical information on the geodynamics of this plate boundary. We infer the state of stress in the PHS slab at the Nankai and Kyushu subduction zones from earthquake distribution and from newly determined focal mechanisms of in-slab earthquakes that occurred during 1997-2002. The majority of the fault plane solutions used in this study were obtained by the Japan Meteorological Agency. Because of significant network improvements since 1997, these new solutions have a better accuracy than those reported previously. At Nankai, the slab has undergone severe deformation, which we attribute to the buoyancy and mechanical incompetence of the young PHS plate and to the immaturity of the subduction zone. Despite the complex slab geometry at Nankai, a simple stress pattern characterized by a general E-W tension and a parallelism of the tension axes with the local strike of the slab surface is observed. The parallelism indicates that the slab is a stress guide. At Kyushu, the older and colder part of the slab experiences down-dip tension due to a greater slab pull. We propose that the margin-parallel component of mantle resistance to the obliquely subducting young slab at Nankai provides an easterly stretch, acting against the westerly slab-pull of the older part of the slab at northern Kyushu. The interaction between the slab in this stress regime and the overriding plate probably contributes to the roughly margin-parallel E-W compression in the overriding plate. Large in-slab earthquakes occur in a high-seismicity band (the Geiyo-western Kyushu (GWK) seismicity band), the northern part of which approximately separates the Nankai region from the Kyushu region. In this band, the slab experiences the maximum E-W stretching and a sharp bending and is thus in a stress environment favoring large in-slab earthquakes. The temperature and pressure conditions at Nankai allow dehydration reactions in both the slab crust and slab mantle. Facilitated by dehydration embrittlement and driven by tectonic stresses, the M 6.7 damaging Geiyo earthquake of 2001 occurred at the northern end of the GWK seismicity band, rupturing both the crust and mantle of the slab. Using a simple elasto-plastic model of bending stress, we demonstrate that rupture of both crust and mantle of the slab can occur under such conditions.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
DE: 7230 Seismicity and seismotectonics
DE: 8100 TECTONOPHYSICS
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting